The present invention instead of relying primarily on a chopper drive, substitutes an improved current-modulated step motor drive.
It provides a more efficient means of control by using the steering transistors as current regulating devices.
Consideration of balancing phase currents (particularly in half-step operating mode) has traditionally been neglected. The present invention provides a method for insuring phase current equilibrium. In this way, a reduction in low-speed cogging as well as an improvement in positional accuracy is achieved.
In the present invention by allowing the chopping action to be a function of the motor's speed, a reduction in torque fall-off can be realized.
To overcome these problems, control techniques employing readily available digital integrated circuits would be desirable.
The current modulated step motor drive of the present invention provides for all of the above-mentioned needs. It is believed that the following features and advantages of this drive are unique.
Complete digital control provides accurate timing and freedom from drift conditions experienced in analog control circuitry.
All control and feedback signals are completely synchronized with the system clock. This allows for properly timed logic state occurrences, eliminating spurious pulses and decreases the chances of "glitches" being interpreted as real logic signals.
A digital speed range detection scheme is incorporated to provide motor velocity information to the drive. This scheme requires no mechanical apparatus and uses no analog circuitry.
Current chopping action is variable and controlled as a function of motor speed. In this way, problems associated with interaction between stepping and chopping are eliminated.
Current level control is maintained as a function of motor speed. This allows for an extension of torque output into the higher speed ranges.
Accurate half-step positioning is obtained by varying current levels in the phase windings depending upon which state the motor is presently in. When in the one-phase energization state, the phase current is doubled over the value of phase current which is allowed for in the two-phase energization state. This allows for an even distribution of stator flux, resulting in smooth rotation and fine positioning ability.
The current feedback scheme utilized in the chopping control is not affected by variations in the storage times of individual transistors. Because the scheme is self-correcting, the components used do not have to be hand selected so as to assure repeatability.
Dual utilization of the output drive transistors is made. In addition to providing the motor phase switching, the current chopping function is also performed by these transistors.
The torque/phase current control scheme provides the framework of the current modulated step motor drive. The velocity-variable time chopper and the dual function drive transistor configuration are integral developments which are incorporated into this control scheme.